Presented at the IEEE/PCA Cement Industry Technical Conference ,Salt Lake City, May 2000 By: Didier Chetelat, Area Sales Manager MAAG Gear AG

1. Introduction

Improvements to cement mill operation via installation of planetary Gearboxes as "replacement kits", have recently proven to be very successful, due to quick return on investments thanks to the immediate achievement of higher efficiency and improvement in mill operation.

The installation of a "replacement kit" gearbox in most cases includes the utilization of the existing foundation by installation of "tailor made" intermediate frame or adaptation to the gearbox casing. With this concept of customized solutions, the cement plant saves time and money, as the erection time of the new "replacement kit" gearbox is limited to a time frame of maximum 2-3 weeks. Cost optimization is further achieved by possible utilization of the existing auxiliary drives as well as no running in time. Finally, no displacement of the main motor is required due to the compact design of the 2-stage planetary gearbox.

The overall improvement in service life for cement mill "replacement kits" gearboxes along with optimization of maintenance cost, is shown based on actual recent installations.

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2. Why a Replacement-Kit (Retrofit) for Ball Mill Drives

There are three reasons why a gear reducer should be replaced

• Replacement of an old gear reducer

Every gear older than 15 years which for spare parts are no longer available, is considered old. Also gears which demand too much time for maintenance. What is to be considered is: how often the customer has to stop the grinding line for the maintenance of gear reducer. Therefore, we can compare the costs of the maintenance of the old gear and the investment for a new one as follows:

Number of service calls per year on the existing gear	Cost A
Cost for manpower for the repair	Cost B
How many spare parts are used per year for the existing gear	Cost C
Cement production lost during the maintenance of the existing gear	Cost D
	Total costs 1 for maintenance

The total cost can be compared to the price of a new gear reducer (Cost E) with an additional 2 weeks for erection (Cost F) and loss of cement production during these 2 weeks (Cost G). The final costs could look as follows:

Total Costs for maintenance of the existing gear	Total Costs 1(above)
Cost for a new gear reducer (Replacement-Kit)	-Cost E
Cost for erection of the new gear	-Cost F
Loss of cement production during these 2 weeks of erection	-Cost G
	Cost difference

• Replacement of an defective gear reducer

Gear reducers can also be replaced because of a technical problems, such as broken teeth (see picture 1.) But mostly, a broken tooth generates other damage, because of the tooth metal particles which pollute the oil. This can generate great damage to the gear wheels and roller bearings. In this case, we make an evaluation of the costs to replace the defective gear wheel, to replace other damaged parts, or to replace the whole gear as described in the chapter 2.1.

Picture 1 Broken teeth

The picture 1 shows 2 broken teeth on a sun pinion of double helical gear. Actually, there were about 12 broken teeth on this sun pinion. No reparation was possible, therefore, there was no other alternative other than to replace the gear reducer.

• Increasing of the power

Another reason to replace a gear reducer is the increasing of the power of the grinding line. It is really seldom possible for gear reducers to accept a power increase over 20% of the initial power. Therefore, customers usually decide to buy a new gear.

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3. Example of a gear retrofit in Africa

The gear reducer replaced in Togo in 1999 is a good example of an old gear which was too expensive for maintenance for the above mentioned reasons.

The picture 2 & 3 shows the old gear reducer that was to be replaced.

Picture 2 Defective Design

In order to be able to quote for a replacement kit, the customer forwarded us basic information about the existing gear reducer environment such as:

• Existing gear reducer general arrangement
• Foundation drawings
• Grinding line arrangement
• Interface sketches (low speed coupling/mill and high speed coupling/main motor

The engineering department defined which gear size was suitable to replace the existing one (1800 kW). Then they redesigned the existing sketches and placed our gear reducer, a new low speed coupling and a new barring device, superimposed on the existing one with the modifications which had to be done to the foundation.

The customer was then able to recognize which modifications would have to be made. As soon as he decided to go this way, a so- called "checklist for heavy duty gears" was sent to him, with the following points to be clarified:

• Condition of environment (temperature, humidity, …)
• Power supply
• Capacity of the customer’s workshop (crane capacity, machine type available, qualification of workers, …)
• Design of the mill building
• Main motor information (power, speed, axial bearings, direction of rotation,…)
• Oil supply unit (cooling water availability and quality,…)
• Couplings (type and design of the existing couplings)
• Mill type (axial bearings, mill flange design,…)
• Auxiliary drive (power, speeds,…)

The second step was to send an engineer on site in order to check all data transmitted from the customer and to guarantee a proper job, because according to our experience, it is a fact that most of the sketches are not the most recent version. It can be that the customer made a revision in the past, or some modifications to the grinding line or foundation without reporting them on the sketches. All components were checked.

• Check of all dimensions in comparison with sketches ?
• Check of the existing equipment: What can be reused?
• Check of the foundations: are they in a good shape (any necessary modifications)?

After this visit, it was obvious that most of the equipment should be replaced, however the foundation was in good condition and only one modification was necessary. Barring devices foundations had to be cut in order to replace it with a new one.

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4. Engineering

In comparing picture 2 (existing gear reducer) with pictures 3 and 4 below, what has been readapted can be seen.

The following chapter will explain what was changed, and which problems were faced.

 



Picture 4 New design

• Planetary gear reducer

Pictures 3, 4 and 5 show a planetary gear which was used to replace an old one, because they are more compact and easier to transport and assemble. There were no major difficulties faced with the main gear reducer, as there was enough available space. No modifications were made to the main gear, a standard one was used. The only difference with the old one was the fixation of the foundation. The old one was transversely fixed and the new one is longitudinally fixed to the foundation.

Sun pinion and planet wheels of alloy steel, tooth flanks case-hardened (carburized) and ground with the necessary longitudinal and profile modifications. Internally toothed annulus made of heat treatable steel, teeth accurately cut. Gear casing made of welded steel plates, oil-tight, with white metal babbitted sliding bearings and pressure lubrication.



Picture 5 Planetary gear

• Intermediate frame

In case of replacement, there are two possible figures regarding the fixation of the new gear in a foreign environment:

1. The casing is redesigned to be directly adapted to the foundation. This solution is used very seldom, because most of the parallel shaft gears which have to replaced, are too large.
2. The second solution is to manufacture a new intermediate frame placed between the gear reducer and the foundation, as shown in picture 6.

The intermediate frame is a simple welded steel frame, which can be adapted to any foundation and assures a perfect stability of the gear reducer.

• Low speed coupling

The low speed coupling was also too old to be reused: however, the standard toothed couplings, which are usually used for this type of job, were too long. It was necessary to shorten it to about half the length. The new coupling was completely redesigned and adapted to the mill with a new intermediate flange shown below in picture 7.

The intermediate flange allows a connection between the mill flange and the toothed coupling and avoids ever having to make modifications to the coupling. All connection holes were pre-drilled in the factory. The finished bore adaptation was directly done on site.

Picture 7 Toothed low-speed coupling

• High speed coupling

Due to a space problem we decided to use an auxiliary drive with integrated high-speed coupling as shown on picture 8. This point is explained more in detail in the chapter 4.5.

Picture 8 Integrated high speed coupling

• Lube station

The old parallel shaft gear design does not have a lube station. It is directly integrated in the gear reducer, and uses a dip-lubrication system. For the planetary gear reducer, an oil tank is used (picture 9 and 10). All monitoring instruments with instrument panel, pumps, filter and cooler are assembled on the oil tank. It was proposed to the customer to install the lube station, lower than the gear reducer as shown on pictures 9 and 10, so that the lube station would be separated from the gear reducer and could be protected from the dust.

Picture 9 Lube station

Picture 10 Positioning of the new lube station

• Auxiliary drive

Usually, the auxiliary drive is placed behind the main motor, flanged to the second shaft. In this case, it was not possible to go this way because the main motor did not have a second shaft.

The auxiliary drive is coupled to the planetary gear by means of a hand- operated clutch. An electrical interlock between the clutch disengagement mechanism and the circuit-breaker of the main motor ensures that the latter cannot be started with the barring clutch engaged.

The auxiliary drive has a hollow output shaft which encloses the high-speed membrane coupling of the main gear reducer. During the normal mill operation the power from the main motor is transmitted through the torsion shaft.

Before the delivery of the gear reducer, the project manager will inform the customer of what he has to prepare on site. This will allow the installation of the gear within 2-3 weeks.

• Make sure that crane capacity is adequate
• Furnish the manpower in order to work 24 hours a day
• Availability of tools
• Finished modifications of the foundations by the customer (if necessary)

The erection is planned as follows:

• Positioning of foundation plates
• Grounding of foundation plates
• Adaptation of intermediate flange
• Preparation of low speed coupling for installation
• Installation of low speed coupling
• Preparation of foundation

Picture 12 Preparation of foundations

• Lifting the gear reduced onto the foundation
• Connecting the gear reducer with low speed coupling
• Alignment of gear reducer
• Grounding of anchor bolts
• Tightening of anchor bolts
• Connecting the gear reducer with auxiliary drive
• Positioning of lubrication unit
• Installation of lubrication oil pipes
• Electrical connection of lubrication unit
• Start-up

6. Summary

It is obvious that in the current market situation, the cement producer has to be competitive. Competitiveness means:

• Low maintenance costs
• High availability of the equipment (ideal is by 99%)

These factors have a direct impact on the price of the cement. For this reason and according the costs comparison shown in chapter 2, the customer will be able to evaluate the best solution for the long term.

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